This invention relates to an image signal-processing system (hereinafter simply referred to as "an imaging system") based on a synthetic aperture technique, and more particularly to an imaging method and an apparatus therefor which are applied to a synthetic aperture radar or an imaging system effecting the reconstruction of an acoustic image by a synthetic aperture technique.
According to an imaging system based on a synthetic aperture technique which is favorably applied to a radar or acoustic imaging apparatus, a beam of radio wave or acoustic wave is emitted through the space toward a foreground subject in a state progressively expanding at a prescribed angle, to detect an image of the foreground subject from the radio or acoustic waves reflected from the foreground subject. The above-mentioned imaging system which assures substantially constant image-resolving capacity regardless of a distance between a foreground subject and an imaging system (hereinafter referred to as "the depth of a foreground subject") is regarded as one of the most effective means proposed in this particular technical field.
One of the conventional imaging system utilizing an ultrasonic beam involves a transducer radiating an ultrasonic beam which is horizontally movable and vertically expansible at a prescribed angle. When a foreground subject (regarded as a spot reflector for convenience of description) is perpendicularly spaced from the beam-ejecting end of the transducer, the spot reflector is horizontally scanned by an ultrasonic wave. A reflection from the spot reflector received by the transducer is converted into an electric signal, which in turn is supplied to a phase detector. The phase detector detects the phase of the electric signal to produce an analog hologram signal. After being digitized, the hologram signal is stored in a corresponding buffer memory.
The image of a spot reflector is reconstructed by the integration of a hologram signal and the corresponding kernel function. A cosine (cos) hologram signal involved in the hologram signals of the spot reflector contains cos waves which are included in an imaginary envelope wave shaped like the cross section of a convex lens, touching the periphery of the envelope wave with different periods. Therefore, the cos hologram signal indicates a striped pattern consisting of a plurality of stripes which are rendered broader toward the center of the pattern and narrower toward both ends thereof. Now assume that among n rows of the data of the cos hologram signal, the first row of data are represented by H.sub.1, H.sub.2, . . . H.sub.m (m: position integer), and data on the corresponding kernel functions are denoted by k.sub.1, k.sub.2 . . . k.sub.n (assuring n&lt;m). Then the product I.sub.1 of an integration of the H and K forms is competed in a manner indicated by the following equation: EQU I.sub.1 =H.sub.1 .multidot.K.sub.1 +H.sub.2 .multidot.K.sub.2 + . . . H.sub.m .multidot.K.sub.m
The result of integration of data stored in the first row of the data is stored in a first row memory area. Integration is carried out in the same manner as described with respect to the data I.sub.2 . . . I.sub.n involving the second to the nth rows. The results of the integration are stored in the memory areas corresponding to the second to the nth rows. The above data-processing method is also applied to other hologram signal such as sine (sin) hologram signals. An image signal of the spot reflector is obtained by synthesizing the results of processing data on the pologram signals. An image signal thus obtained comprises a main lobe projecting broadly at the center and a plurality of narrow side lobes continuously extending in opposite directions from both sides of the main lobe. For the reconstruction of the spot reflector image with a distinct contrast and at a high resolution rate, the best method is to reduce the difference between the amplitudes of the main lobe and particularly a first side lobe immediately adjacent to the main lobe as much as possible and also to decrease the width of the main lobe at a prescribed decibel level (namely, increase the sharpness Q of the main lobe) to the greatest possible extent. With the conventional image signal-processing system, the amplitude of the first side lobe has a theorical ratio of about -13 dB at best to that of the main lobe. Further, let it be assumed that the image-resolving power is defined to mean the width of the main lobe at a level of -10 dB; the depth Z of the spot reflector (the perpendicular distance of the reflector from the transducer) indicates 15 mm; and an ultrasonic beam at said depth Z has a width of 75 mm. Thus the resolving power of the conventional signal-processing device is only about 1.6 mm. Therefore, elevating the resolving power of the conventional signal-processing device is required.